It looks like some form of insulation with a cardboard backing. The previous tenant in the apartment said he found a stack of this material in the basement and used it to insulate around a window ac unit. Its a two-story building in brooklyn. I'm living in the lower level which I'm told used to be the servant's quarters, so I believe the building is quite old. It looks like it has a had at least a couple renovations. Photo of the material:

I think linatics is correct in that it's probably not asbestos, however you really can't tell just by looking at it. There are some materials (vermiculate insulation being the most well known) that were known to be contaminated in some cases - so even though it was not asbestos, depending on where it came from, it might actually contain it.

If you are unsure, the safest thing to do is have it tested and treat it as contaminated until you find out otherwise. This means not disturbing it other than to take a sample, and make sure you wear a respirator when you do.